1. Field of the Invention
The present invention relates to a ring system and, in particular, to a ring control node which increases the upper limit of the number of nodes that can be arranged on one ring in a BLSR (Bi-directional Line Switched Ring) system utilizing optical transmission devices (nodes), and conforms to the increase in line capacity and the scale of systems accompanying recent technical innovations.
2. Description of the Related Art
The BLSR control method in a ring system is based on the North American standard SONET (Synchronous Optical Network: standard GR-1230-CORE). In a duplex ring line within a BLSR ring system, only a single directional ring is normally used to perform data transfer from a transmitting node to a receiving node. On the other hand, if a fault occurs within the line, data continues to be transferred by switching to the undamaged ring in the opposite direction.
FIGS. 1A and 1B show examples of a ring system using the prior art BLSR method. FIG. 1A shows an example of how the system operates under normal operating conditions, and FIG. B shows an example of how the system operates when a fault has occurred.
During normal operation as shown in FIG. 1A, data sent from the transmitting node 11 is received by the receiving node 14 through, in this example, a counter-clockwise route via node 16 and node 15. When a line fault occurs between nodes as shown in FIG. 1B, line switching is executed based on the APS (Auto Protection Switch) protocol for BLSR in adjacent nodes 11 and 16 enclosing the span (the space connecting nodes) which includes the line where the fault has occurred. In the present example, the node 11 located on the data transmission side of the above-mentioned span bridges the transmission route to a clockwise route, while the node 16 located on the data reception side switches and sends the data received by the clockwise route to the original counter-clockwise route.
FIG. 2 shows an example of a K1/K2 byte format in a SONET main signal line overhead (SOH). The K1/K2 byte format is used in route switching controls and alarm displays, and is based on the APS protocol for BLSR.
In FIG. 2, the four bits 5 to 8 of a K1 byte are assigned to the receiving node ID, and the four bits 1 to 4 of a K2 byte are assigned to the transmitting node ID. Consequently, 16 nodes can be specified for each of the receiving node and transmitting node. Also, the switching request type is set in bits 1 to 4 of a K1 byte; for example, if “1011” is set, this specifies a Signal Fail-Ring Switch (SF-R) request.
If the route bit 5 of a K2 byte is set to “0”, this sets the short path to the receiving node via the ring direction whose route is shortest, and if it is set to “1”, this sets the long path via the ring direction whose route is longest. Further, the node switching status type is set in the three bits 6 to 8 of byte K2; for example, if “010” is set, a bridge and switch (Br&Sw) state is specified.
FIG. 3 shows an example of prior art node ID allocation based on APS protocol for BLSR.
As shown in FIG. 3, node IDS “1” to “8” are assigned to each of the node 21 to 28. Each of the nodes 21 to 28 maintains a topology map so as to recognize all of the other nodes 21 to 28. In the present example, a fault has occurred in the clockwise ring line in the span between node 21 and node 22. In this case, in the adjacent nodes 21 and 22 enclosing the span, firstly the node 22 on the data receiving side detects the occurrence of a fault. Node 22 refers to the topology map and recognizes that the other adjacent node enclosing the span is node 21, sets the receiving node ID “1”, the transmitting node ID “2” and the Signal Fail-Ring Switch (SF-R) request in the switching request in the above-mentioned K1/K2 bytes, and outputs a switching request to both the counter-clockwise (E to W) short path (path bit “0”) and the clockwise (W to E) long path (path bit “1”).
If the receiving node 21 receives the same Signal Fail-Ring Switch (SF-R) request via both the long path and the short path, the switching request and fault location are verified and the path switching process is executed therefrom. Thereafter, the communication route for when a fault occurs is set as shown in FIG. 1B. Note that intermediate nodes 3 to 28 other than the receiving node 21 support fault recovery by pass-through operations.
Using the BLSR control method in this way, when operating normally each of the duplex ring lines can be used for separate data transmission, and since a so-called reserve type or standby type redundant structure is unnecessary, a ring system with high line usage efficiency can be constructed. In recent years, in optical line networks, with increases in line capacities and the scale of network structures accompanying rapid accelerating technical innovation, the demand for BLSR control systems is increasing and their application in large scale ring systems is being eagerly expected.
However, in the prior art BLSR ring system there are the following problems. The first is that, because the transmitting node ID and the receiving node ID are each specified by 4 bits (#0 to 15) in the K1/K2 bytes, it has had the limitation that only a maximum of 16 nodes can be installed on a single ring. As a result, in the prior art, where a network ring of more than 16 nodes has been constructed, an interconnection system (GR1230) or the like between common rings, known as ring interconnection, has been used.
In such a case, a BLSR control used within one ring can be troublesome, and there is the problem that, since it becomes necessary to introduce a new device to interconnect each of the rings, the network equipment and network management costs increase significantly. As a result, it is impossible to capitalize on the advantages of improving the line usage efficiency of the BLSR structure and to satisfy the customers' strong demand to be able to support a wide area with one ring.
Secondly, if the scale of a network is enlarged and the number of nodes installed within one ring is increased, the time taken from detection of a fault till execution of the path switching operation increases in proportion to the number of nodes. As a result, a new problem occurs in that fault recovery cannot be achieved within a suitable time frame. In this case, it is necessary to realize an increase in the throughput speed of the path switching request signal in the increased intermediate nodes other than the receiving node.
Thirdly, in the usage of a topology map by way of BLSR control, there is the possibility of the following problem occurring under certain conditions.
FIGS. 4a and 4B show an example of a case where a mismatch occurs in a topology map.
In the example given in FIG. 4A, the topology map of node 32 starts from its own node ID “2” and is erroneously set in the order “2341”. In this case, it is possible for node 32 to detect the error in its own topology map by means of the receiving signal (#1/S) via the short path from node 31 (ID1), as long as the ring is operating correctly. In this manner it is possible for only the receiving side node 32 to detect a mismatch in its own node ID, then normally the node 32 which has detected the error outputs a mismatch alarm or the like, and the operator performs a topology mismatch recovery operation (correcting it to “2341”).
Next, a worst case scenario wherein the mismatch state in FIG. 4A occurs simultaneously with a line fault will be considered. In such a case, if a fault (indicated by an “x”) occurs in the clockwise ring line as shown in FIG. 4B, node 32 refers to the topology map “2341” without detecting that there is a mismatch, and transmits a path switching request (#4/L) via the same clockwise long to the receiving node 4. Similarly, it transmits a path switching request (#2/S) via the counter-clockwise short path.
In this case, because node 31 directly receives the path switching request via the short path from the adjacent node 32 (ID2) bordering the faulty span, it thereafter waits to receive the same path switching request via the long path. On the other hand, since node 34 (ID4) receives the path switching request via the long path, it thereafter waits to receive the same path switching request via the short path. As a result, the path switching conditions are never realized in either of the node 31 or node 34, the ring system remains in a receiving standby state, and the mismatch alarm is not generated, therefore this causes major problems.